Source for highly stripped ions

ABSTRACT

Apparatus for producing highly stripped ions by sufficient exposure of these ions to a cloud of energetic electrons, for producing an electrostatic negative potential well capable of confining these ions during the stripping process, for creating magnetic and electrostatic forces capable of confining the energetic electrons forming the electron cloud with only a low rate of electron loss and a slow dissipation of electron energy, and contemplating means for producing a transverse electric field that removes electrons undesirably trapped in electrostatic positive potential maxima.

United States Patent 72] Inventor Thomas H. Stix Rehovot, Israel [21 1Appl. No. 856,337

[22] Filed Sept. 9, 1969 [45] Patented Oct. 5, 1971 The United States ofAmerica as represented by the United States Atomic Energy Commission[73] Assignee [54] SOURCE FOR HIGHLY STRIPPED IONS 5 Claims, 11 DrawingFigs.

[52] US. Cl 317/4, 313/63 [51 Int. Cl HOSb [50] Field ofSearch313/61,63,

[56] References Cited UNITED STATES PATENTS 3,421,035 1/1969 Brubaker313/63 3,441,756 4/1969 Janes etal. 310/11 Primary ExaminerLee T. HixAttorney-Roland A. Anderson ABSTRACT: Apparatus for producing highlystripped ions by sufficient exposure of these ions to a cloud ofenergetic electrons, for producing an electrostatic negative potentialwell capable of confining these ions during the stripping process, forcreating magnetic and electrostatic forces capable of confining theenergetic electrons forming the electron cloud with only a low rate ofelectron loss and a slow dissipation of electron energy, andcontemplating means for producing a transverse electric field thatremoves electrons undesirably trapped in electrostatic positivepotential maxima.

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' THOMAS H. STIX PATENTEU DDT 5I97l 3,611,029

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SOURCE FOR HIGHLY STRIPPED IONS BACKGROUND OF THE INVENTION Thisinvention, made in the course of, or under a contract with the UnitedStates Atomic Energy Commission, relates generally to ion sources, andmore particularly to an improved source for producing highly strippedheavy ions. One method for producing ions comprises the well-knownPhilips Ionization Gauge (P.I.G.) for producing ions in a Penning typedischarge, as described on p. 152 et seq. of Controlled ThermonuclearReactions" by Glasstone and Lovberg, Van Nostrand, i960, and p. 90 etseq. of "Particle Accelerators by Livingston and Blewett, McGraw Hill,1962. Basically, this system comprises a tubular anode interposedbetween two spaced apart cathodes at the opposite ends of the anode andmeans for producing a steady state magnetic field parallel to the axisof the tubular anode. In operation ions continuously form with some ofthe ions being accelerated back to the perforated cathode where theyproduce secondary electrons and the magnetic field prevents thesesecondaries from immediately reaching the anode whereby the secondaryelectrons oscillate back and forth between the two cathodes to producemore ions. Heretofore, however, these devices have lacked provision forion confinement, have produced lowenergy protons, have had highdissipation, or have contained large numbers of neutral particles. It isadditionally advantageous to provide fully stripped heavy ions abovehydrogen in the periodic table for use in the fields of low-energyphysics, medical application, or acceleration in conventionalaccelerators.

SUMMARY OF THE DISCLOSURE This invention provides an improved ion sourcefor producing highly stripped ions by their sufficient exposure to acloud of reactively confined energetic electrons and for confining theseions in an electrostatic negative potential well during the period oftheir exposure and for ejecting them therefrom when desired. Moreparticularly, in one embodiment this device comprises means forproducing an axial static magnetic field and, with spaced apart cathodesand anodes, for producing a double-humped electrostatic potentialprofile parallel to the magnetic field such that energetic electronsare, with low particle loss and with low dissipation, confinedtransversely by the magnetic field and axially by the electricpotential, which is almost everywhere positive with respect to thecathodes, and also such that ions are confined transversely bytheelectric field due to the negative space charge of the cloud ofelectrons thus confined, and confined axially by the electric potentialminimum in between the two positive electrostatic humps. In anotheraspect, this invention provides means for producing a transverseelectric field that removes electrons undesirably trapped in positivepotential maxima and also ineans for producing a slab-shaped cloud ofenergetic electrons between said anodes for ionizing said ions in saidelectrostatic well profile. With proper selection of components andparameters as described in more detail hereinafter, the desired ions areproduced and confined in said well and ejected therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sketch of potential in themodified P.I.G. geometry of this invention. The potential, P, is plottedversus the distance z, parallel to the magnetic field. Ions are confinedby the potential I I electrons are confined by the potential-(d ,,-d

FIG. 2(a) and FIG. 2(b) are sketches of equipotential lines, D constant,in a plane perpendicular to the magnetic field. Two possibleconfigurations are illustrated. Shaded region represents conductingboundary. Potential well is created by the space charge of the electroncloud.

FIG. 3(a), FIG. 3(b) and FIG. 3(a) are sketches of possible electrondensity and potential profiles in the anode region of a P.l.G. device.The density profile in (b) and (c) is broadened by the diocotroninstability. Limiting the cathode emission eliminates the virtualcathode in (c).

FIG. 4 is a sketch of potential versus 2 showing drift'fields forremoving trapped electrons. Potentials along lines of magnetic forcenear top-center and near bottom-center of the modified P.I.G. device aredepicted.

FIG. 5(a) and FIG. 5(b) are cross sections, perpendicular to B, ofequipotential surfaces in the clearing field region. Sketch (b) showsthe configuration when the additional E field is sufficiently strong toopen up all the equipotentials. In (a), corresponding to a smallerimposed field, a portion of the electron cloud potential well remainsundestroyed.

FIG. 6 is a sketch of electric potential, D, versus 2 showing thecontributions to D from the space charge of the electron and ion clouds.Of particular interest is the reduction of height in the ion-confiningbarrier.

FIG. 7 illustrates a partial, small scale, three-dimensional view of oneembodiment of the modified P.I.G. device of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT This invention hasparticularly utility in providing a source of highly stripped ions,comprising heavy ions, for the production of transuranium elements. Tothis end, this invention provides an ion source for conventionalaccelerators that accelerate the ions against suitable targets, such ascomplex nuclei. However, the ion source of this invention also providesions for any of a wide variety of other applications, comprisinglow-energy and high-energy physics, and/or medical applications. TheIEEE Trans. on Nuclear Science NS-l4," No. 3, 7(1967), has discussed thedifficulties of heavy nucleus acceleration, and the Berkley group hadproposed a new-device utilizing injection, acceleration, foil-strippingand reacceleration to achieve a desirable output energy. However, it hasbeen pointed out regarding the HIPAC system of U.S. Pat. No. 3,441,756,and in Dubna Preprint P7-4l24 (1968), that a high degree ofelectron-stripping may be obtained by confinement of heavy ions in theelectrostatic potential well of an electron plasma, and by the exposureof these ions to the energetic electrons in thisplasma. The impressivesignificance of this concept is that the availability of a proper sourceof prestripped ions would make possible the efficient acceleration ofheavy nuclei in conventional accelerators, such as cyclotrons,synchrotrons, of Van de Graaf brand electrostatic accelerators.

A description of the entrapment of positive ions in the negative spacecharge of a cylindrical electron beam appears in Elec. Commun. 24, 108(1947). The experimental results described herein comprise observationof enhanced positive ion trapping when biasing electrodes at each end ofa drift tube so as to create a pair of small (-l5v.) positive potentialhills. Further reports on this concept appeared in Proc. Inst. RadioEngrs. 42, 1,548 (1954) and J. Appl. Phys. 26, 1,157 (1955) where, withthe addition of an axial magnetic field for collimating the electrons,the description reports the prevention of longitudinal ion drain fromone or both ends of a positive potential barrier. The thrust of thesereports, however, comprises the reduction of the electrostaticdispersion in focusing the electron stream inside various electron tubedevices.

A second group of reports on ion sources for mass spectrometers,describes multiple ionization in a source having ion trapping, bothaxially and radially, by the space potential of a magneticallycollimated electron beam. Also, Can. J. Phys. 45, 1,79l (1967) describesion charge multiplicity up to Z =XE from a mass spectrometer source withboth magnetic collimation and with a pair of ion barrier electrodes.

A common feature of the above-mentioned references comprises thecollection of electrons at an anode. biased at or very close to themaximum positive potential. However, the cross sections for strippingbecome small at high 2 values, and to achieve reasonable stripped-ionproduction rates requires a large flux of electrons in the 5-10 k.e. v.range. If then, as described in the above-mentioned references, theionizing electrons make only a single transit through the trapped-ionregion and then strike the anode with their maximum velocity, thesesystems have an extremely low efficiency of production, and reasonableproduction rates may require megawatts of power.

On the other hand, the toroidal geometry of the above-mentioned I'IIPACsystem of US. Pat. 3,441,756 comprises the reactive storing of theelectron energy, thereby to prevent dissipation except by atomic orcooperative plasma processes. In accordance with this invention,therefore, a linear multipletransit trapped-ion source reactively storesthe electron ener- 8)" To this end, as in the above-mentioned I-IIPACsystem where a high degree of electron-stripping occurs by ionization ofheavy ions by the energetic electrons in this plasma, the apparatus ofthis invention utilizes this significant feature. However, in accordancewith this invention, this energetic plasma provides both ion confinementand stripping. To this end, whereas the l-IIPAC has a toroidalconfiguration and employs a rising magnetic field for electroninjection, this invention employs a modified P.I.G. configuration ofimproved geometry, having a longitudinally extended slab-shaped electroncloud, end injection, and a steady-state magnetic field. This inventionthus has significant differences from both the HIPAC system, andconventional P.I.G. devices.

In understanding how the reactive trapped-ion source of this invention,referred to as an rti source, employs slab geometry, end injection, andsteady-state magnetic field, as well known in the P.I.G. art, thisinvention replaces the conventional anode collector in a P.I.G. by areflector electrode biased approximately to the cathode potential, thusreactively to confine the electron plasma in a modified PhilipsIonization Gauge configuration. Moreover, this invention modifies thelatter by introducing a pair of electrodes to create a doublehumped,ion-confining electrostatic potential profile. Additionally, since theseaxial ion barriers tend to trap electrons so as to become reduced instrength, this invention adds a transverse electric field to removethose electrons trapped in the ion-confining potential maxima. Also, asdescribed in more detail hereinafter, the modified P.I.G. geometry ofthis invention has particular quantitative relations for theelectrostatic potentials; in one embodiment utilizes the diocotroninstability, and establishes a desired ionization rate. Additionally,the following discusses the relaxation of the electron distributionfunction, the gradual neutralization of the ion-confining potentialmaxima through the accumulation of the trapped electrons, and method andapparatus for ejecting the trapped electrons by EXB drift. Finally, thefollowing discusses the accumulation of the trapped ions and ionejection mechanisms. To aid in this discussion, the following provides adescription of one embodiment ofthis invention.

As understood in more detail hereinafter, this embodiment forms an "rtisource envisioned as an electrostatic potential well for ionconfinement. To this end, electrodes external to the ion plasma create adouble-humped electrostatic profile which confines the ions in theirmotion parallel to the magnetic field, while the space charge of amagnetically confined electron plasma produces an ion-confining wellperpendicular to the magnetic field. In turn, negatively biased endplates, as in the conventional PIG. geometry, confine the electronsparallel to the magnetic field.

Referring now more particularly to apparatus 1 of FIG. 1, thisembodiment has a vacuum electric potential as a function of distanceparallel to a conventional, static, magnetic field, such as in a P.I.G.device. In this illustration of FIG. 1 herein, end plates 3 and 5,negatively biased with respect to a central region 7, locate in a staticmagnetic field produced by means 9, and produce a double-humpedelectrostatic potential profile 11. The region of ion confinement of thesystem of FIG. 1 thus comprises a potential well 11 between twopotential humps l3 and 15.

Ions in the regions exterior to the potential humps l3 and 15 fallquickly into the negative end plates to provide an ion population oftransiting particles within these exterior regions, and this populationcomprises transiting particles only. However, the electron population inthe exterior regions corresponds substantially identically with theinterior electron population in potential well 11. Therefore,space-charge effects of the unneutralized electron cloud in the exteriorregions place a limit on the charged particle density, wherein a crudeestimate of the allowable plasma radius (e.g., the plasma sheetthickness) simply corresponds to the Debye shielding distance for kT e(I ;I

FIGS. 2a and 2b illustrate possible shapes for the equipotential surfacein a plane perpendicular to the magnetic field for a cylindricalconfiguration, and in the preferred rectangular embodiment of thisinvention, the latter providing a slab-shaped electron cloud. Forconstant uniform density of electron space-charge, an increase incylindrical radius in the configuration ll of FIG. 2a, proportionallycorresponds to thickness, approximately independently of the width.Thus, the same well depth of the device of FIG. 2b tends to hold moreelectrons per unit length than the charge cylinder 11 of FIG. 2a.Accordingly, the following discusses the charge slab embodiment 1 ofthis invention having a slab width much greater than the slab thickness.In this regard, for ease of explanation, the following discussionpertaining to the charge slab embodiment 1 employs two-dimensional(infinite width) approximations where appropriate. Thus, for example,the depth of the potential of the well 11 of FIG. 2b, perpendicular tothe magnetic field, for a uniform density electron space charge, isapproximately 1rrz, (z)ea' 7 2 where "8(2) equals the electron densityin cm. and d equals the slab thickness in cm.

To stabilize long wavelength (small k instabilities, sometimesclassified as diocotron or slipping stream instabilities, in a uniformlydense slab of electrons in a uniform magnetic field, the device of FIG.2b utilizes suitably conducting walls. Short wavelength instabilitieswith a growth rate proportional to the square of the electric fieldperturbation at the top of the slab due to a rippled boundary along theslab bottom, i.e., mi exp-2kzd, classified in the same category, alsooccur. These short wavelength instabilities may have wavelengths withDoppler frequency between ripples drifting with EXB velocity along theslab top and bottom is greater than the electron cyclotron frequency,that is,

k,Av=k,(Etop-Ebottom)C/B 2 eB/mw. Accordingly, differentiation of eq.(1) yields the electric fields. Substitution for k, then yields 0), exp(2/q) or in more detail,

MZ) =2.Z6 10 ml volts (1) with B in gauss. Since the diocotron growthrate of eq. (2) has enormous sensitivity to the value of the electronplasma, the plasma q-criterion advantageously corresponds to from q 0.07to 0.12.

The combination of equations (1 and (3) provides a scaling law for therri source 1 of this invention having the doublehumped potential profile11 of FIG. 2b. The total rate of ionization comprises:

where N, equals the total number of ions of charge j, n, equals theaverage density of ions of charge j and y), equals the ionization ratefactor. The active volume equals the slab area, A, times the thickness,d. Elimination of n, and d yields with I in volts, Bin gauss, av in cm./sec., and A in cm.

This slab geometry of this invention also provides for the ejection ofelectrons undesirably trapped in potential maxima. In this regard, theelectrons undergo collisions with ions and other electrons and tend tothermalize. Reaching isotropy in velocity space alone robs theseelectrons of two-thirds of their initial energy parallel to the magneticfield, and thus strongly traps them desirably inside the anode volume.Further collisions produce electron velocities closely approachingMaxwellian distribution, and the electron density thus 'tends to vary asexp(e(z)/kT, This thermalization proceeds on a time scale given byelectron self-collision time, approximately seconds each sheetdetermined by scrape-off to an aperture-limiting protuberance. FIG. 3aillustrates this density profile and the pertinent potential variation,while recognizing a possible thickness of the sheets as small as theLarmor diameter of the electrons at the cathode temperature. However,the diocotron instability will tend to broaden out these thin sheets,FIG. 3b. Furthermore, while the high electron density of the twin-thinsheets configuration of FIG. 3a tends to raise the ionization rate, thelow kinetic energy of the electrons operate as a deleterious factor viareduction of (o'v To avoid this condition, one may rely not only on thetfiocotron instability to raise the average electron kinetic energy butcan also consider limiting the cathode emission to diminish the depth ofthe space charge well 1 1. FIG. 30 illustrates a potential profile thateliminates the virtual cathode in this manner.

However, collisional relaxation that tends to increase n, in regions ofmaximum 1 i.e., tending toward n,=exp[e l 9z OlkT 1 still limits thefavorable density and potential profile of FIG. 1. As a specificexample, electrons that undergo collisions in the vicinity of thepotential maxima can lose enough parallel energy to remain trapped inthese regions. Ionization additionally frees trapped electrons in thesesame locations. Moreover, in the modified P.I.G. configuration of FIG.1, the space charge of the trapped electrons lowers the height of thepotential maxima and accordingly reduces the ability of thatconfiguration to confine ions.

Also, the rate of entrapment of electrons by collision equals (t,.) ineq. (6), and the production of free electrons via ionization in theregions of the potential maxima obeys a rate equation in the form of eq.(4). Thus, these two processes can destroy the ion-confining potentialmaxima by filling the same with trapped electrons in a time comparableor short compared to the desired ion confinement time. This inventiontherefore, thus advantageously provides for the removal of the trappedelectrons from the Plasma while retaining those electrons that executefull cathode-to-cathode oscillations.

To this end, in accordance with the described embodiment of thisinvention, a sideways EXB drift ejects electrons trapped in the twoion-confining potential maxima l3 and 15. Thus, for example the E fieldin the x-direction superimposes on the existing electric field in eachof the regions of the potential maxima, but in opposite directions, asillustrated by FIG. 4. Electrons executing full end-to-end oscillationsdisplace oppositely in the y-directions as they pass through the tworegions of the potential maxima of the double humps 13 and 15 of profile11. Thus, electrons transversing the slab device I of this invention,sum up to zero displacement to close the orbits of the electronoscillations. However, the electrons trapped in one end of the slabdevice I experience drift displacements always in the same direction dueto the described EXB drift thereby to transport and eject electrons'from the sides of the device 1 for collection by suitably placedcollecting electrodes biased at high potentials.

Advantageously, the drift E field exceeds a predetermined minimumstrength, since without this E drift, the equipotential lines of theplasma take the shape represented by the drawing of FIG. 2b, while thespace charge of the electron cloud produces the potential well in thissection perpendicular to B. Since the electrons drift in the EXBdirection in their motion perpendicular to B, their trajectories, tendto follow such equipotentials. Thus, as illustrated in FIG. 5a, a modestE field does not open up all the closed equipotentials thereby tendingto leave trapped the electrons drifting along the closed lines.

FIG. 5b illustrates the equipotentials when a sufficiently strong Efield opens up all the closed lines, thereby permitting all the trappedelectrons to drift out of the plasma 19 in central region 7. For anelectron cloud of uniform density from the top to the bottom wall, thepotential to make E, have the same sign everywhere, equals 4 A b, i.e.,

top bottam where A I equals the depth of the well 11, in the absence ofthe clearing field, due to an electron cloud of the same density. Also,the charge density of the untrapped electrons is reduced in the regionof increased potential due to the higher velocity of the particlesthere.

While the removal of trapped electrons by the clearing field couldlocally produce a double-humped electron velocity distribution, andthereby tend to produce a two-stream instability mechanism, the shortz-extent of the trapping volume, the velocity shear v,(x) due to theclearing field, and the velocity spread of the untrapped electronsadvantageously suppresses this instability.

In a practical device for producing a slab geometry in accordance withthis invention, there prevails a small ionization cross section ofalready ionized heavy nuclei, typically 10 cmF, thus requiring a longion confinement time, typicallyz 0.1 second for the desired highlystripped states. The following, therefore, examines the process for theion formation and ion loss.

Electron impact causes the ionization. On each such impact the potentialenergy of the ion changes by the amount e d (the electron acquiring -e 1potential energy), and the total energy acquired by an ion equals e Z I(r,) where r, equals the position at which the jth ionization stepoccurred. The average ion energy per stripped electron equals one-thirdthe depth of a parabolic well 11 with homogeneous ionization. Theaverage ion kinetic energy per stripped electron, therefore, equalsone-sixth the well depth, with the energy divided among the threedegrees of freedom for ion motion.

Ions escape from the potential well 11 with their kinetic energy in theA? direction exceeds the potential barrier height at that point.However, escaping ions carrying an average amount of energyperpendicular to AQ will transport at least l0eA I /9 per strippedelectron, or ten-thirds times the average total energy I s 0) for theion plasma. Therefore, the remaining ions have proportionally lesskinetic energy and lie deeper in the potential well 11.

By this means, the collisional ion loss mechanism involves only thetail" of the ion distribution with a self-limiting loss rate due to theselective nature of the process. More importantly in the practical sensethe accumulation of positive charge weakens the confining potential well11. In FIG. 6, which illustrates the potential variation in one possiblesteadystate condition, the uppermost curve represents the vacuumelectric potential along the centerline of the modified P.I.G. device 1of this invention having a double-humped potential profile 11. Thelowermost curve illustrates the electrical potential added to theelectron space-charge effects. The further addition of ion space-chargeproduces the middle curve, with the deviation of this middle curve fromthe cathode potential depending on the extent of limitation of thecathode current. The presence of ion space-charge tends to increase thepotential in the central region 7, and thus reduces the relative heightof the potential barriers of well 11. Also,

the ion-confining well 11 becomes shallower with diminished volume.Moreover, although ions form and reionize in the well 11, their lossover the potential barriers at the ends of the device I, as reduced inheight, balances their accumulation rate.

Restating the situation another way, an increasing rate of ion formationincreases the positive component of the spacecharge in the centralregion 7 represented in FIG. 6, thereby reducing the effective barrierheight and increasing the rate of ion end loss. On the other hand, anincreasing electron emission lowers the net potential in the centralregion 7 but also lowers the height of the potential in the regions ofthe ion confining barriers 13 and 15, with the net results of verylittle change of barrier height.

In this regard, judicious adjustment of the ion loss process controlsthe extraction of stripped ions from the rti source 1 of this invention.Accordingly, this device 1 produces a steady stream of ions out the endsthereof. However, the device 1 can alternately provide pulsed operation.To this end, a negative pulse applied to one of the potentialhump-producing electrodes causes the ion-confining barrier temporarilyto lower to release a cloud of stripped ions. Pulsing just a section ofone of these electrodes focuses the stream of ejected ions.

The small test device 1 shown in FIG. 7 has an iron-core magnet 21 thatproduces a 10,000 gauss magnetic field between pole pieces 23 and 25forming a -inch gap. The reflectors 27 have a 5.6 cm. separation, andthe ion-confining region 7 has 4.6 cm. length, a 3 cm. width and a 0.3cm. thickness. The anode electrodes 29 have a kv. potential, oppositelydirected and kv. clearing fields are applied to electrodes 37.

An 0.08 cm. wide hot tungsten ribbon 24 oriented in the xdirection,emits into the plasma through an 0.05 cm. by 0.3 cm. slot 33 in thereflector electrode 27 for a plasma 19 having a corresponding fullthickness. The above-described EXB drift carries the electrons along theequipotentials, as shown in FIG. 2, thereby filling the volume insidethe rectangular electrodes. Adjustment of the bias voltage on thefilament 24 and the control grid 47 between the filament 24 and thereflector 27 controls the injection current into the active volume 7.The outer anode pairs 37 of the five separate anode electrodes 29 placedbetween the two reflector electrodes 27, provide the double-humped,ion-confining potential profile l1 and the clearing field. The innerthree anodes 39 produce the central field in central region 7, whilegiving some flexibility in adjusting the shape of the central potential.

A bakeable ultra-high vacuum is provided in a suitable enclosure 41around the device 1. Also provision is made for steady-state orintermittent pulsed operation of the clearing field, as well as thecontrol grid 35 and one of confining anodes 29 for studying the buildupand decay of the plasma, and to dump the plasma.

The diagnostics rely on measuring the electrode currents understeady-state and pulsed operation, and on studies of the intensity andapparent source region of the spectral lines emitted from the differentcharge states. These diagnostics in combination with the negativepulsing of the one of the confining anodes 29, allow the ions to passout end 43 of the device I and through a hole 45 in the pole piece 23 toan analyzer (not shown for ease of explanation).

Advantageously also, careful control over the gas supply of neutral gasto the plasma volume controls the ion formation process. Likewise, the msource device 1 of this invention requires careful control of theemission from filament 24, and has a control grid 47. With this control,the spatial extent of the plasma l9 fills the rectangular area betweenthe cross section of anodes 24. Adjustment of the bias voltage on thefilament 24 and on the control grid 47 between the filament 24 and nearthe reflector 27 controls the injection current into the active volume 7of the plasma 19. Additionally, biasing a pair of suppressor electrodes51 adjacent each reflector 27 drives back secondary electrons releasedfrom the electrode surfaces of reflector 27. Next come the outer anodepairs 37 that produce the ion-confining potential maxima and theclearing fields, while the three middle anode pairs 39 set the potentialof the control region 7. Also, although the diocotron and two-streaminstabilities can tend to exist, and the removal of trapped electrons bythe clearing fields can produce locally a doubled-humped electronvelocity distribution, the abovedescribed control helps provide theshort z-extent of the trapping volume, the velocity shear v,(x) due tothe clearing field, and the velocity spread of the untrapped electronsadvantageously to suppress the latter instability.

With control of the two-stream instability and control of the buildup ofthe electron density, comprising control of the diocotron instability,as described above, then eq. (2) for a q of 0.03 gives w,,,=l0 years,and the corresponding electron density at B=l0,000 gauss equals 3 10emf. Likewise, for a slab thickness of 0 0.3 cm. eq. (1) provides a welldepth of 6,100 volts. Additionally, a 10 kv. potential on the central ofthe anodes 39, provides a mean electron energy of about 5,900 ev.,corresponding to an RMS electron velocity of 4.5 l0 cm./sec. In thisexample, the surface through flux of electrons corresponds to a currentin each direction of about 60 amperes per cm."'.

From the above described example, for A=l0 cmF, Zn n=0.l, and a=l0 cm.eq. (4) then corresponds to a production rate of l.2 l0"Z ions persecond, equivalent to an ion current" of 0.2 microarnperes. Followingtherefrom, the average ionization time, with a'=l0 cmF, equals 75milliseconds, and the electron-scattering time from eq. (6), equals 25milliseconds. Using this latter time as approximate to theelectron-trapping rate, then, an injection current of 6p. amperessufiices to replenish the ejected trapped electrons. The comparison ofthe injection current to the 60 ampere/cm. energetic electron flux inthe central plasma region 7 corresponds to the effectiveness of theelectron confinement.

While the above has described an embodiment for producing thedouble-humped electrostatic potential well of this invention in a slabgeometry, as herein understood in the art from this description, variousother embodiments will come within the preview of this invention, aslikewise understood in the art from the description herein.

Also, the described and/or other embodiments of this invention, asunderstood in the art, may provide the described EXB apparatus forremoving trapped electrons or other like apparatus for performing thesame function.

This invention has the advantage of providing a doublehumpedelectrostatic profile for producing and reactively confining ions in animproved P.l.G. configuration. To this end, the invention provides aslab geometry, provides novel EXB means for removing trapped electrons.Accordingly, this invention has the advantage of providing an improvedP.l.G. means for producing and confining highly stripped ions,comprising heavy ions, for a wide variety of applications, such as theproduction of transuranium elements, for high-energy and/or low-energyphysics, and/or for a wide variety of medical or other applications.

I claim:

1. in an ion source of the type having a gas source, means for producinga magnetic field and a cloud of energetic electrons in said field forproducing ions from a gas from said source, the improvement comprisingmeans forming a doublehumped electrostatic potential profile immersed insaid field for confining said electron cloud and said ions forrelatively long periods of time for producing highly stripped heavy ionsfrom a gas from said gas source.

2. The invention of claim 1 having means for producing a transverseelectric field for removing electrons undesirably trapped in positivepotential maxima of said double-humped electrostatic potential profile,thereby to prevent said electrons from accumulating therein so as toneutralize the ionconfining operation of said positive potential maxima.

3. The invention of claim l in which the electrodes are so disposed andbiased so as to cause the individual injected lOl0O4 0567 5. Theinvention of claim 1 in which each hump of said double-humpedelectrostatic profile substantially corresponds in potential strengthwith the other corresponding hump of said double-humped electrostaticprofile.

1. In an ion source of the type having a gas source, means for producinga magnetic field and a cloud of energetic electrons in said field forproducing ions from a gas from said source, the improvement comprisingmeans forming a double-humped electrostatic potential profile immersedin said field for confining said electron cloud and said ions forrelatively long periods of time for producing highly stripped heavy ionsfrom a gas from said gas source.
 2. The invention of claim 1 havingmeans for producing a transverse electric field for removing electronsundesirably trapped in positive potential maxima of said double-humpedelectrostatic potential profile, thereby to prevent said electrons fromaccumulating therein so as to neutralize the ion-confining operation ofsaid positive potential maxima.
 3. The invention of claim 1 in which theelectrodes are so disposed and biased so as to cause the individualinjected energetic electrons advantageously to make numerous transitsthrough the ion cloud.
 4. THe invention of claim 1 in which said meansforming said double-humped electrostatic potential profile has a slabgeometry for producing a slab-shaped electron cloud in said profile. 5.The invention of claim 1 in which each hump of said double-humpedelectrostatic profile substantially corresponds in potential strengthwith the other corresponding hump of said double-humped electrostaticprofile.